The present invention relates to improvements in a hydraulic shock absorber having telescoping inner and outer hollow cylindrical bodies wherein the inner cylindrical body is provided with a radial partition wall through which a piston rod axially extends from the top wall of the outer cylindrical body.
In order to facilitate a thorough understanding of the advantageous functioning of the shock absorber of the present invention as described in detail hereinbelow, an exemplary shock absorber construction is schematically illustrated in FIGS. 17-19 of the accompanying drawings so as to emphasize some of the disadvantages which might be encountered in a shock absorber which does not incorporate all of the features of the present invention. Such exemplary hydraulic shock absorber illustrated in FIGS. 17-19 includes a piston rod which extends through a hole formed in a partition wall dividing an enclosed cylindrical space, and is adapted to provide a hydraulic damping force at the clearance between the piston rod and the partition wall in either a contraction or an extension stroke.
As shown in FIGS. 17 and 18 the exemplary hydraulic shock absorber comprises outer and inner cylinders 1 and 2, respectively. The inner cylinder 2 has an upper partition wall 3 and a lower partition wall 4 provided with radially spaced oil paths 4a. The outer cylinder 1 has a piston rod 5 axially extending from the top wall thereof and penetrating holes 3a and 4b formed respectively in partition walls 3 and 4. The piston rod 5 is provided at the lower end thereof with a piston 6 provided with radially spaced oil paths 7 and with a slidable check valve 8, there being a clearance provided between check valve 8 and a radially outer edge portion of each oil path 7.
In such type of hydraulic shock absorber, with the descent of the piston 6 during the compression stroke, oil in a lower section S.sub.1 lifts up the check valve 8 and flows through the paths 7 into a first intermediate section S.sub.2 and thereafter through the paths 4a into a second intermediate section S.sub.3. The oil further flows through the clearance S.sub.5 defined between the piston rod 5 and the hole 3a of partition wall 3 into an upper section S.sub.4 as shown in FIG. 17. In such flow of oil the rate of flow is controlled by the clearance S.sub.5, and a damping force in the compressing direction is obtained in such portion. The clearance S.sub.5 functions in effect as an orifice.
During the extension stroke on the other hand, oil in the upper section S.sub.4 is caused to flow through the clearance S.sub.5, through the oil paths 4a in lower partition wall 4, through the paths on the outer periphery of check valve 8, and into the lower section S.sub.1 as shown in FIG. 18, to provide a damping force.
With such shock absorber, the clearance S.sub.5 serves as an oil path during both the compression and the extension strokes. Because clearance S.sub.5 provides the same sectional area for both strokes, it is difficult to obtain a satisfactory damping force characteristic in the extension stroke. In practice, different damping characteristics are required for the compression stroke and for the extension stroke. However, with the above-described shock absorber in which the rate of oil flow is controlled by the same clearance in both the compression and the extension strokes, assuming that satisfactory performance can be provided for either one of the two strokes, unsatisfactory performance can be expected in the other stroke.
In the event that a variable orifice is provided by using a piston rod 5 as shown in FIG. 19, such piston rod 5 being tapered or having a sectional area which varies continuously over its length, the clearance S.sub.5 is progressively reduced and the damping force is progressively increased as the piston rod descends in the compression stroke, so that a satisfactory damping characteristic can be obtained. In the extension stroke, however, with the relative ascent of piston rod 5, the sectional area of the clearance S.sub.5 is increased. Therefore, the damping force is quickly reduced, causing the inner and outer cylinders 1 and 2 to be quickly extended. As a result, sudden tossing or impelling forces in the extending direction are felt. Thus, it is difficult to obtain a satisfactory damping function in the extension stroke.
With such construction, which includes a variable orifice between the piston rod 5 and hole 3a by providing the tapered piston rod, satisfactory performance can be obtained in only one of the two strokes and it is difficult to obtain satisfactory damping performance for both the compression and extension strokes.
The present invention effectively solves the foregoing problems associated with known hydraulic shock absorber constructions, while at the same time providing a simplified construction having excellent shock-absorbing qualities.